Mobile station with improved channel quality prediction for wireless communications

ABSTRACT

A wireless transmit/receive unit (WTRU) with improved performance through channel quality prediction employing link adaption techniques including a receiver which makes selective measurements on downlink transmissions, and then stores one or more of the measurements or a channel quality indicator derived therefrom. The receiver then retrieves one or more of the past measurements (or the past channel quality estimates themselves), and combines it with current measurements (or the current channel quality estimate), to predict what the channel quality will be at some future time and derive a predictive channel quality indicator (CQI). This predictive CQI, derived from both current channel measurements and at least one past channel measurement, is then sent to the transmitter for use in updating transmission parameters.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/698,721, filed on Oct. 31, 2003 now U.S. Pat. No. 7,912,490, whichclaims priority from U.S. patent application Ser. No. 60/423,620, filedon Nov. 1, 2002, which is incorporated by reference as if fully setforth.

FIELD OF INVENTION

The present invention generally relates to wireless communicationsystems. More particularly, the present invention is a method employedby a wireless communication system for improved channel qualityindication in dynamic link adaption.

BACKGROUND

Various algorithms are currently used by present wireless communicationsystems for estimating channel quality at a wireless receiver. Thesealgorithms are employed, for example, in systems using the ThirdGeneration Partnership Project (3GPP) High Chip Rate Time DivisionDuplex (TDD) mode, the 3GPP Low Chip Rate TDD mode, the 3GPP FrequencyDivision Duplex (FDD) mode, the time division—synchronous code divisionmultiple access (TD-SCDMA) standard, and High Speed Downlink PacketAccess (HSDPA) extensions of the aforementioned systems. The qualityestimates may be used for transmit power control, in- andout-of-synchronization decisions, radio link failure decisions, andchannel quality indicators (CQIs) to support dynamic link adaptation,(e.g., adaptive modulation and coding (AMC)) techniques.

In the TDD mode for instance, the quality indicator, referred to as CQI,sent by the User Equipment (UE) on the high speed-shared informationchannel (HS-SICH) is a recommended Transport Format Resource Combination(TFRC). In general, the TFRC refers to the possible transport blocksizes, modulation schemes, and any other link adaptation parametersavailable. The recommended TFRC is usually based on the signal mostrecently received by the UE.

Regardless of whether or not the communication system is a 3GPP system,the CQI could represent a recommended Transport Block Size, modulationformat, number of codes, power offsets, or any one of a number ofdifferent types of link adaptation parameters. These CQIs are derived bya receiver and signaled to a transmitter to set the transmissionparameters for a subsequent transmission.

The CQI typically provides either specific link adaptation information,such as a recommended coding and modulation scheme for the AMC function,or provides one or more general quality indicators which aresubsequently used to base the selection of appropriate transmissionparameters.

If the CQI is not accurate, the selected modulation and coding scheme(or other transmission parameters) will be suboptimal. Overestimatingchannel quality can cause the UE and Node B to continue attempting touse a modulation and coding scheme when reception quality is too poor tojustify their continued use. Underestimation of channel quality may leadto excessive transmission power and inefficient use of radio recoursesor, in the case of in- and out-of-sync processing, ultimately apremature declaration of radio link failure and release of radioresources. Thus, a call may be dropped without cause. Excessivetransmission power will, in turn, lead to a system-level throughput losssince interference in other cells may increase needlessly. Accordingly,inaccurate channel quality estimation reduces throughput, wastestransmit power, and increases interference to other cells.

A shortcoming of prior art channel estimation techniques is that sincethe techniques estimate channel quality at a receiver, they do notprovide sufficiently accurate estimates of channel quality at thetransmitter at the time of the subsequent transmission. Referring toFIG. 1, a prior art CQI generation and reporting procedure 100 between aUE and a Node B is shown. The Node B transmits a message on a downlink(DL) control channel (step 102), informing the UE which resources havebeen allocated to the UE for the next associated DL data transmission.The UE receives the control message regarding the allocation ofresources and awaits the receipt of the DL data transmission (step 104).

The Node B sends the associated DL data transmission (step 106). The UEreads the DL data transmission (step 108) and makes selective qualitymeasurements (step 110). Using the measurements from step 110, the UEderives a CQI (step 112) that it estimates would provide the highestthroughput, while still meeting other possibly specified requirements,such as a block error rate (BLER).

The UE then reports the most recently derived CQI to the Node B in thenext available UL control channel (step 114). The Node B receives theCQI (step 116) and then uses the CQI to set the transmission parametersfor the next data transmission (step 118).

There are drawbacks with the current method of providing CQI feedback.For example, the current 3GPP specification does not set a specific timelimit on how long the UE may take to derive the CQI. This could take aninordinately long time. It is, however, required (and desirable) thatonce the CQI is derived from the given data transmission, it is reportedin the next available UL control channel. This minimizes the delay ingetting the CQI information to the Node B. However, even if the delay ingetting the CQI information from the UE to the Node B is minimized, thedelay is not eliminated.

As shown in the example timing diagram of FIG. 2, there is a CQImeasurement period on one or more DL transmissions, during which the UEmakes selective measurements on the DL transmission. As shown, themeasurements may be performed on a DL data channel, a DL pilot channel,or a combination of both the DL data and pilot channels. After themeasurements are performed, the CQI is calculated; this is shown at timet₁. Although the delay is minimized by reporting the CQI to the Node Bat the next available UL transmission (shown at time t₂), there isadditional delay until the subsequent use by the Node B of the CQI(shown at time t₃) to set the parameters for the next downlink datatransmission.

The delay (graphically designated as A) between the completion of themeasurements upon which the CQI is based (at time t₁) and the subsequentuse by the Node B to set the associated transmission parameters at timet₃ results in a CQI that is not accurate by the time it is used by theNode B. The greater this delay, the less accurate the CQI becomes. Asthe CQI becomes less accurate, the DL channel quality will ultimatelysuffer since the transmission parameters will be based on a CQI thatdoes not accurately reflect the true channel conditions. In essence, theprior art methods of CQI determination reflect the past conditions ofthe channel.

It would be desirable to provide a method of channel qualitydetermination without the severe disadvantages of known prior artsystems.

SUMMARY

A wireless transmit/receive unit (WTRU) with improved performancethrough channel quality prediction employing link adaption techniquesincluding a receiver which makes selective measurements on downlinktransmissions, and then stores one or more of the measurements or achannel quality indicator derived therefrom. The receiver then retrievesone or more of the past measurements (or the past channel qualityestimates themselves), and combines it with current measurements (or thecurrent channel quality estimate), to predict what the channel qualitywill be at some future time and derive a predictive channel qualityindicator (CQI). This predictive CQI, derived from both current channelmeasurements and at least one past channel measurement, is then sent tothe transmitter for use in updating transmission parameters.

BRIEF DESCRIPTION OF THE DRAWING(S)

A more detailed understanding of the invention may be had from thefollowing description of preferred embodiments, given by way of exampleand to be understood in conjunction with the accompanying drawingwherein:

FIG. 1 is a flow diagram of a method for CQI generation and reporting inaccordance with the prior art.

FIG. 2 is a timing diagram showing the delay associated with the priorart CQI reporting method of FIG. 1.

FIG. 3 is a predictive CQI generation and reporting method in accordancewith a preferred embodiment of the present invention.

FIG. 4 is a predictive CQI generation and reporting method in accordancewith a first alternative embodiment of the present invention.

FIG. 5 is a predictive CQI generation and reporting method in accordancewith a second alternative embodiment of the present invention.

FIG. 6 is a timing diagram showing the elimination of the inherent CQIdelay associated with the embodiments of the present invention shown inFIGS. 3 and 4.

FIG. 7 is a graph showing the distribution of the difference between theCQI generation and reporting process in accordance with the prior artand the predictive CQI generation and reporting process in accordancewith the present invention.

This application uses the following acronyms:

-   -   3GPP Third Generation Partnership Project    -   AMC Adaptive Modulation and Coding    -   CDMA Code Division Multiple Access    -   CQI Channel Quality Indicator    -   DL Downlink    -   FDD Frequency Division Duplex    -   HSDPA High Speed Downlink Packet Access    -   HS-DPCCH Shared Information Channel for HS-DSC (FDD)    -   HS-SICH High Speed Shared Information Channel for HS-DSCH (TDD)    -   SIR Signal-to-Interference Ratio    -   TDD Time Division Duplex    -   TD-SCDMA Time Division-Synchronous Code Division Multiple Access    -   TFRC Transport Format Resource Combination    -   UE User Equipment    -   UL Uplink

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention provides an improved method of channel qualityprediction without the disadvantages of the prior art.

Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a UE, mobile station, fixed or mobile subscriber unit, pager,or any other type of device capable of operating in a wirelessenvironment. Each of these terms may be used interchangeably herein.When referred to hereafter, a Node B includes but is not limited to abase station, site controller, access point or any other type ofinterfacing device in a wireless environment. Each of these terms may beused interchangeably herein.

It is to be noted that the present invention is applicable to TDD, FDD,TD-SCDMA, CDMA 2000, and other modes and types of transmissions withoutexception. More generally, the present invention is applicable to anycommunication system employing a scheme which monitors channel qualityand adapts the transmission parameters of subsequent transmissions basedupon the channel quality, such as AMC or other forms of radio linkadaptation.

In accordance with the present invention, the CQI is a predictiveindicator of the quality of future channel conditions. While either aNode B or WTRU may perform such predictions, the present invention willbe described hereinafter as being performed at the WTRU. Additionally,although the invention will be described as a receiver performingmeasurements and deriving the CQI, it is equally possible for thereceiver to perform the measurements and transmit the measurements tothe transmitter which then derives the CQI. It would also be understoodby those of skill in the art that the present invention is equallyapplicable to the uplink (UL) or DL transmissions, such as in the caseof link adaptation in the UL, where the roles of the WTRU and the Node Bas described hereinafter will be reversed.

In a slotted system where the transmission bursts may span several timeslots, interference levels in these time slots can vary greatly. Thepresent invention recognizes that channel fading conditions may changesubstantially from slot to slot. By allowing (but not requiring) CQIprediction on a per slot basis, the prediction of channel quality can beimproved. The channel quality reported to the transmitter can thereforebe made more accurate, compared to the prior art situations.

Referring to FIG. 3, a procedure 200 for generating and reporting a CQIin accordance with the present invention is shown. The procedure 200 isinitiated by the Node B transmitting a downlink control messageregarding the allocation of resources to the WTRU (step 202). The WTRUreceives the control message regarding the allocation of resources onthe downlink control channel (step 204). The message informs the WTRU ofthe timing of a subsequent data transmission, and of the transmissionparameters of the subsequent data transmission (for example, the type ofmodulation, coding, etc.). The Node B then sends a downlink datatransmission to the WTRU (step 206) which is received by the WTRU (step208). The WTRU makes selective CQI measurements regarding the downlinkdata transmission (step 210), derives the current CQI (step 212), andthen determines a predictive CQI (step 214). As part of step 214, theWTRU stores one or more of the CQI measurements and/or the CQI for lateruse in determining the predictive CQI. Additionally, it should be notedthat it is not necessary to derive a current CQI in order to determinethe predictive CQI. Thus, step 212 could be considered optional in thisembodiment. For example, past CQI measurements may be combined withcurrent CQI measurements to derive a predictive CQI.

The predictive CQI is derived from both current measurements and atleast one past measurement. The WTRU retrieves one or more of the pastCQI measurements (or the past CQI themselves), and combines it with thecurrent CQI measurement (or current CQI), to predict the quality offuture channel conditions.

In one embodiment of the present invention, the prediction method usedin step 214 to derive the predictive CQI is the Linear Predictionmethod. This is a well known mathematical technique for predictingfuture values based upon the combination of current and pastinformation. The Linear Prediction method minimizes the prediction errorin the least squares sense. In a preferred embodiment, thesignal-to-interference ratio (SIR) expressed in dB is the quantity beingpredicted. However, other factors may be included, such as prediction ofsignal power and noise power separately. Other prediction methods can beused, and may be selected with both performance and minimizingcomplexity in mind.

After the predictive CQI is derived at step 214, the WTRU reports thepredictive CQI to the Node B (step 216) and the Node B receives thepredictive CQI at step 218. The Node B then uses the predictive CQI toset transmission parameters for the next transmission (step 220).

It should be understood by those of skill in the art that certain stepsmay be combined depending upon the specific implementation of thismethod. For example, as shown in an alternative embodiment of a method400 of the present invention in FIG. 4, steps 210, 212, and 214 may becombined into a single step 408 for determining the predictive CQI. Allother steps in FIG. 4 remain the same as the steps described withreference to FIG. 3.

Additionally, as shown in FIG. 5, steps 202 and 204 need not be part ofthe procedure 500, whereby the WTRU automatically receives the DL datatransmission without a prior DL control message.

Whether the specific process for determining the CQI is set forth inseparate steps 210-214 as shown in FIG. 3 or a single step 408 as shownin FIG. 4, it would be understood by those of skill in the art that, incontrast to the prior art methods of CQI determination which reflect thepast conditions of a communication channel, the present inventionderives a predictive CQI which predicts the future conditions of acommunication channel. The present invention makes current measurements,but predicts and reports to the Node B a predictive CQI which estimatesfuture channel conditions. As aforementioned, this predictive CQI isderived from both a current CQI measurement or current CQI derivedtherefrom and at least one past CQI measurement or past CQI derivedtherefrom that has been stored. The predictive CQI estimates the qualityof the channel conditions closer to the time the Node B is ready totransmit.

Although the CQI is shown as being derived from only a single datachannel, the UE may use the DL data transmission (of step 206), anyavailable pilot signals, or combinations of both to derive the CQI.

In accordance with the preferred method 200 of the present invention,the predictive CQI will be much more likely to reflect the actualchannel conditions that the Node B will experience when it is ready tosend another transmission, rather than a CQI measurement that isreflective of a past transmission, as shown in FIGS. 1 and 2.

Referring to FIG. 6, although the WTRU makes the current CQI measurementat the same time as the prior art scheme (at time t₁), and then combinesit with the prior CQI measurements for transmission to the Node B at thesame time as the prior art scheme (at time t₂), the WTRU in accordancewith the present invention predicts what the channel condition will beat time t₃. In the example illustrated in FIG. 6, the “apparent” CQIdelay vanishes since the CQI has been predicted to line-up in time withthe DL data channel. Accordingly, when the Node B is ready to transmitthe DL data (at time t₃), there is no delay (shown as B=0), since thepredictive CQI that was sent is a CQI that was predicted at time t₃.

Even if there is a delay between the completion of the CQI measurements(at time t₁) and the use of the measurement by the Node B, this delaywill be shorter than the delay A shown in FIG. 2. By using availablepast information about the channel quality history, the reported CQI canbe computed to reflect the channel quality that will exist at the timeof the next DL data transmission, thereby making the selected code rate,modulation type and other link adaption parameters more accurate.

Although FIG. 6 shows the CQI measurements being performed on both theDL data channel and the DL pilot channel, it would be understood bythose of skill in the art that the CQI measurements may be performedsolely on a DL data channel, solely on a DL pilot channel, or performedon a combination of both the DL data and pilot channels.

Although there will also be an associated error in the predictive CQImeasurement (since it is predicted and not actual), this error is likelyto be smaller than the prior art method of sending an outdated CQImeasurement. FIG. 7 shows how using the prediction scheme used inaccordance with the present invention can be employed to improve thereporting accuracy of channel quality conditions at the time of theactual transmission, thereby improving the preference of any dynamiclink adaption systems. In FIG. 7, a distribution of the differencebetween the SIR measured and the SIR at the time the SIR is used isshown. In this example, the delay is 10 msec.

There are two probability distribution curves shown, one for the priorart method of sending a CQI measurement based on past channelconditions, illustrated as curve A, and the second for the currentmethod of sending a predictive CQI measurement based upon a futurechannel condition, illustrated as curve B. With the present invention(curve B), there is a higher likelihood that an associated error will besmaller, and a lower likelihood that an associated error will be larger,than with the curve A of the prior art method. The distribution for theprediction signal in accordance with the present invention is moreconcentrated near zero error than the delayed signal of the prior art,indicating that the CQI reporting errors are smaller when usingpredictive CQI.

Although the present invention has been described in detail, it is to beunderstood that the invention is not limited thereto, and that variouschanges can be made therein without departing from the scope of theinvention, which is defined by the attached claims.

1. A wireless transmit/receive unit (WTRU) comprising: a receiverconfigured to receive a downlink data communication via a downlinkcommunication channel; a processor configured to perform selectivechannel quality indication (CQI) measurements on the downlink datacommunication to determine a predictive CQI, wherein the predictive CQIestimates a future quality of the downlink communication channel on aper time slot basis and includes at least one of a recommended transportblock size, modulation format, or number of codes; and a transmitterconfigured to transmit the predictive CQI for setting transmissionparameters for a future transmission.
 2. The WTRU of claim 1 furthercomprising a memory, wherein the WTRU stores at least one of theselective CQI measurements.
 3. The WTRU of claim 2 wherein the processoris configured to derive a current CQI and store the current CQI in thememory.
 4. The WTRU of claim 3 wherein the processor is configured toretrieve at least one of the CQI measurements from the memory andcombine the at least one retrieved CQI measurement with the current CQIto determine the predictive CQI.
 5. The WTRU of claim 1 wherein theprocessor is configured to determine the predictive CQI using a linearpredictive algorithm.
 6. A method of a wireless transmit/receive unit(WTRU) predicting a future quality of a downlink communication channel,the method comprising: receiving, at the WTRU, a downlink datacommunication via the downlink communication channel; the WTRUperforming selective channel quality indicator (CQI) measurements on thedownlink data communication to determine a predictive CQI, wherein thepredictive CQI estimates a future quality of the downlink communicationchannel on a per time slot basis and includes at least one of arecommended transport block size, modulation format, or number of codes;and the WTRU transmitting the predictive CQI for setting transmissionparameters for a future transmission.
 7. The method of claim 6 furthercomprising storing at least one of the selective CQI measurements in amemory of the WTRU.
 8. The method of claim 7 wherein the WTRU derives acurrent CQI and stores the current CQI in the memory.
 9. The method ofclaim 8 further comprising: the WTRU retrieving at least one of the CQImeasurements from the memory and combining the at least one retrievedCQI measurement with the current CQI to predict the quality of futureconditions of the downlink communication channel.
 10. The method ofclaim 6 wherein the predictive CQI is determined using a linearpredictive algorithm.